A method for press shaping and hardening a sheet-steel workpiece having a small material thickness and good dimensional accuracy is known from GB 1,490,535. Here the steel blank is heated to a temperature above AC3, then pressed in less than 5 sec into the final shape between two indirectly cooled shaping tools while being deformed, and then rapidly cooled, while remaining in the press, to obtain a martensitic and/or bainitic fine-grained structure. A hot-formed part produced this way is used in vehicle construction, for example, for structural and safety parts such as bumpers and B-columns. Hereinafter, the above-described shaping and hardening of a workpiece that has been heated to over AC3 while remaining in a press is defined as hot shaping. Hot shaping can be carried out both with a flat sheet and a previously cold-formed blank.
So as to cut a part produced this way with good dimensional accuracy, however, there are considerable problems. In particular, cold-cutting hardened materials (hard cutting) requires very high cutting forces, resulting in rapid tool wear and high maintenance costs. Furthermore, the cold cutting of high-strength parts is problematic since the cut edges have some burrs, which may quickly cause cracking in the part due to the high notch sensitivity of the high-strength materials. A high-strength part can be cut only to a limited extent, or not at all, depending on material thickness.
So as to avoid the difficulties occurring during the mechanical cutting of hardened parts, frequently alternative cutting methods are used, such as laser cutting or water jet cutting. While these methods allow high quality trimming of the part edge, these cutting methods operate relatively slowly since the cycle times depend directly on the length of the cutting edge as well as the required tolerances. Hence US 2006/137779 of Brodt therefore proposes trimming the edges of a blank that largely corresponds to the contour already in the unhardened state of the part. This is intended to avoid trimming of the part after hardening by the hot-shaping process. However, this is only possible if the tolerances of the part permit such a method.
Furthermore, WO 2005/075279 of Dreher proposes to locally reduce the material thickness in certain areas of a structural part made of sheet metal in that the part is provided with a locally defined stamping during hot shaping. However, this method only serves to adapt two or more parts in terms of material thickness such that the parts can be joined in a controlled manner. This method therefore does not take the above-described trimming problem of high-strength parts into consideration.